WO2015082292A1

WO2015082292A1 - Testing of an industrial structure

Info

Publication number: WO2015082292A1
Application number: PCT/EP2014/075763
Authority: WO
Inventors: Bastien CHAPUIS
Original assignee: Commissariat A L'energie Atomique Et Aux Energies Alternatives
Priority date: 2013-12-02
Filing date: 2014-11-27
Publication date: 2015-06-11
Also published as: EP3077795B1; ES2773545T3; US10324026B2; EP3077795A1; FR3014200A1; FR3014200B1; US20160377528A1

Abstract

The present invention relates to a method and device for analysing a structure by tomography and ambient noise cross correlation. An optical fibre comprising a plurality of fibre Bragg grating (FBG) measurement points comprising Bragg grating (BG) sensors is placed in or on the structure to be analysed. The method comprises emitting light, into the optical fibre, and taking a correlation measurement for each pair of FBG sensors. In one development, an initial image of the structure is obtained by reconstructing propagation speeds. Other developments comprise: determining the positions of the FBG sensors; calibrating the tomography; the configuration of the rosette of sensors forming the measurement points; and the use of a plurality of optical fibres, multiplexers, lasers, optical circulators, omnidirectional optical sensors and active sources of noise (such as piezoelectric transducers, whether integrated or not into the structure).

Description

CONTROL OF INDUSTRIAL STRUCTURE

Field of the invention

The invention relates to the field of metrology and in particular that of the health control of structures using optical fibers.

State of the art The control of the integrity of the structures (structures, planes or pipelines for example) during their life is generally done during maintenance operations, with inspection and human intervention. A concrete technical problem consists, for example, in detecting and dimensioning a corroded area on an aircraft fuselage.

For these integrity checks, so-called non-destructive (NDT) control methods are generally used, according to so-called "conventional" methods (by ultrasound, according to electromagnetic methods, etc.). In recent years there have been research developments aimed at integrating sensors into the structure at key points of the structures, in order to automate the measurements (for example at regular intervals, these intervals being generally close together in time) and to be able to access information on the state of health of certain inaccessible areas, without dismantling or interrupting the functioning of the structure. In general, these developments aim to space maintenance intervals, and thus save money. In particular, some researches envisage the use of guided ultrasonic waves (OG) emitted and detected by piezoelectric transducers (for example PZT type) integrated into the structure. These guided waves propagate over a large distance (a few tens of cm to a few hundred meters in very favorable geometries such as pipelines), so that a limited number of transducers can control a large area. Other technologies can be used to emit and / or detect guided ultrasonic waves (in addition to optical fibers, PVDF films or magnetostrictive sensors for example).

A general technical problem lies in finding an acceptable compromise between the number of sensors to integrate (cost, size, weight, etc.) and the quality of the information recoverable by these sensors. A high number of sensors means a high cost and a low number of sensors often implies a lack of reliability of the information, risks of false alarms, or a lack of redundancy in the event of a sensor failure. The multiplication of the sensors, however, poses other specific problems (for example each integrated sensor may constitute an additional point of weakness, which could lead to new defects in the structure). For each sensor, it is also necessary to provide power supply son, which is not always possible. In industrial practice, very few applications provide a satisfactory compromise.

As regards the nature of the sensors, the known solutions of the state of the art using lasers as measuring systems are not usable in all circumstances. In particular, lasers can not be integrated into structures. Some known approaches consist in carrying out a reference measurement of the structure in the healthy state in order to notice a difference with a subsequent state to reveal the presence of the defect. In order to make this operation more reliable, different signal processing techniques exist, in particular to compensate for the influence of the temperature, but none is really effective. In all cases the interpretation of the signals remains very delicate.

The various aspects of the invention overcome these disadvantages, at least in part.

Summary of the invention

Certain embodiments of the invention advantageously provide for the use of Bragg gratings, in particular "FBG sensors" or "FBG sensors" or "FBG measurement points", "FBG" being the acronym for the English expression " Fiber Bragg Grating ".

A Bragg grating (or distributed Bragg reflector) is a quality reflector used in waveguides, for example in optical fibers. It is a structure in which layers of two different refractive index materials alternate, which causes a periodic variation of the effective refractive index in the guide. A Bragg grating is a submicron modulation of the refractive index of the core of the fiber: a network of a few millimeters thus comprises several thousand steps. From a functional point of view, it plays the role of a reflector for a fine spectral band centered at a characteristic wavelength proportional to the pitch and index of the core of the fiber. Thus, any modification of these parameters proportionally shifts the Bragg wavelength. The tracking of its spectral displacements makes it possible to go back to the inductive parameters, like the temperature or the deformations undergone locally by the optical fiber. These Bragg gratings are made by laser within the heart of single-mode fibers. The inscription of these networks can in particular be carried out by transverse insolation with an interference pattern created by two laser beams.

According to one embodiment, there is disclosed a method of analyzing a diffuse field correlation structure, an optical fiber having a plurality of measurement points, a measurement point comprising Bragg grating type sensors (Fiber Bragg Grating, FBG), the optical fiber being deployed "in" (for example "placed a posteriori" or "integrated natively in") or "on" (for example "placed on" or "attached" or "associated with") the structure to be analyzed, the method comprising light emission in the optical fiber; and the correlation measurement for at least a portion of the FBG sensor pairs of the acoustoelastic field propagating "in" (or "within", "through", "via") the structure. Sensors or pairs of FBG sensors can be interrogated substantially simultaneously. By the term "substantially", reference is made to the speed of elastic waves and to the fact that metrologically the interrogations occur in a time delta (close time intervals to obtain significant measurements from the point of view of the propagation of elastic waves in the structure). All or some of the sensors can be interrogated, according to various implementations. A subset of sensors may be interrogated substantially simultaneously, while another subset may be subject to a delayed interrogation (for example sequentially or in parallel manner in pairs or even combine these modes of operation. interrogation, rotation, etc.). The acousto-elastic field refers to the field of mechanical waves (sound, ultrasound, etc.) that propagate in a solid medium. Unlike the case of the fluid, there are two types of acoustic waves for a solid material. These waves are better known as elastic waves (shear and compression-traction). The acoustoelastic effect reflects a dependence of the propagation velocity of the acoustic waves as a function of the state of deformation of the solid. The structure to be analyzed does not involve any particular restrictions insofar as any type of structure (particularly industrial) can be analyzed by the methods and systems presently described.

In a development, the method further comprises a step of reconstructing the propagation velocities by tomography, the imaging being carried out by inverting all the flight times between the FBG sensors, each flight time for each pair of FBG sensors being deduced from the correlation measurement. This development is optional. It has the advantage of improved subsequent interpretation.

In a development, the position in the space of each measuring point is previously and individually measured. This solution has the advantage of its simplicity of implementation. In a development, the temperature of the structure is measured and a variation in flight time induced by a temperature change is compensated. The temperature can indeed affect the flight times and it is appreciable to be able to correct or compensate for the thermal effects. Concretely, a thermocouple can be used but other methods of measurement are possible.

In one development, the imaging of the tomographic structure is performed by measuring at least two flight times, a first measurement being made in an initial or reference state and a second measurement being performed in a later state (for the same pairs of measuring points). The subsequent state is called "current", it therefore corresponds to the present time of the measurement ("second measurement set").

The realization of a tomography (entirely optional) on the data resulting from the first measurement makes it possible in particular to identify certain geometrical particularities of the structure. This corresponds to a static measurement made in the initial (or reference) state. The imaging of the structure at rest can be possibly subtracted from subsequent images of the structure (subtraction of pixels, that is to say in terms of image content). In other words, this optional mapping makes it possible to identify certain geometrical features of the structure so as not to confuse them with defects on the mappings obtained on the second measurement set. In one development, the method further comprises a second measurement performed in a later state for the same pairs of measurement points as the first measurement and further comprises a tomography mapping of propagation velocity variations in the structure between the state. initial state and the subsequent state obtained from differences in flight times measured between the two states. In other words, the variation of the flight times (measured for the couples) between the two states is measured. This makes it possible to obtain, by tomography, a mapping of the elastic (elasto-acoustic) wave propagation velocity variations between the moment of the measurement associated with the current state (present) and the moment associated with the reference state. . For example, between the moment f of the measurement and the initial time t ₀ , we can see that the waves go "slower" (respectively "faster") in some places and deduce the identification of defects or damage. caused to the structure. In a development, a measurement point includes an FBG sensor. The use of a sensor of this type to make diffuse field correlation has not been described a priori. In another development, a measurement point comprises three receivers and directional FBG sensors substantially disposed at 120 ° from each other in a so-called rosette configuration. The rosette configuration is the compromise that minimizes the number of hardware elements while ensuring a good quality of measurement. A measurement point may also include any number of FBG sensors (eg, 5 sensors, 6 sensors, etc.).

In one development, correlation measurement includes correlation coda correlation between FBG sensors. This entirely optional development optimizes the device, since it makes easier the arrangement of the optical fiber on the structure. As a result, the time required for implementation can be reduced, the positioning errors of the measurement points minimized, etc. The "correlation coda correlation" consists, for a pair of measuring points A and B, of choosing any measurement point C, selected from the set of measuring points (except A and B); correlating the measurements for each of the points A and B with this arbitrary measuring point C ,; correlating the coda of these correlations to obtain the correlation between measurement points A and B. It is possible to repeat the operation for some or all of the possible measurement points C and to sum the correlations obtained to obtain a correlation between A and B with better reliability. All this can be applied to all or some of the possible pairs of FBG sensors.

In one development, it is disclosed the use of a plurality of optical fibers, each having (at least partially sensors FBG). Implementations in practice may vary. Each sensor or pair or pair of sensors can be interrogated separately.

There is also disclosed a system for analyzing a structure, comprising at least one optical fiber having a plurality of measurement points, a measurement point comprising one or more Bragg grating type sensors (FBG). ; a light source coupled to the optical fiber; a photodetector or an optical spectrum analyzer for analyzing the reflected light after it has traveled through the optical fiber; and signal processing means for performing correlation and tomography calculations.

In one development, the source of the light source is a wavelength varying laser or a broadband optical source whose reflected optical spectrum is determined. Lasers are now common and the associated measurements are performing well.

In one development, the optical fibers may be multiplexed using, for example, optical circulators and / or spectrum analyzers and / or multiplexers.

In one development, FBG-type unidirectional sensors are complemented or replaced by omnidirectional {Doppler effect-based Fiber Optic (FOD) sensors. The sensors can therefore all be of the FBG type, or all of the FOD type, or else the method can be implemented on a system comprising both types of sensors simultaneously (in variable proportions, versus economic and performance aspects). In a development, the system further comprises one or more active noise sources positioned in or on the structure of in order to obtain a diffuse acoustoelastic field, that is to say, at best respecting the characteristics of a diffuse field. In one embodiment, said placement or positioning is interactively guided by the measurements in progress. In another embodiment, the location of the noise sources is determined theoretically (i.e., "predetermined"). In another mode of implementation is returned an indication as to the adequacy of said positioning (versus the diffuse field hypothesis). In another implementation, the multiplicity of noise points or sources (combined with randomly placed placements) tends to guarantee the obtaining of a diffuse field (without return loop, ie a priori). In other words, there is disclosed a system comprising one or more active noise sources that can be used in addition to or in replacement of the natural sources of noise present in the structure, which may also be advantageous for calibration. These additional sources may be for example piezoelectric transducers judiciously placed in the structure, in order to be able to make measurements when desired (for example, in an airplane if the natural sources are turbulence in flight, it will advantageously make use of additional active sources to perform a measurement on the ground, when there is no more "natural" noise in the structure). These sources will advantageously be placed in such a way as to create an acoustic field that best respects the equi-energy distribution condition. For example, to satisfy this condition, the sources can be placed close to natural diffusers (or even around the area to be controlled). It is possible to use active sources integrated into the structure. It is also possible to use active sources that are not integrated: for example a jet of compressed air whose contact zone randomly sweeps the structure to be examined (so as to satisfy the condition of equi-distribution in energy). The system can therefore comprise at least one source of noise, said source being example a jet of compressed air whose contact area randomly sweeps the structure.

In a development, an active noise source may be a piezoelectric transducer, possibly of the PZT type.

According to one aspect of the invention, a large number of measurement points is advantageously used in combination with a diffuse field correlation measurement, which has never been done with FBG sensors, in order to perform tomography.

According to a particular embodiment of the invention, the structure to be examined is "imaged". This imagery allows easier interpretations than those resulting from the analysis of raw signals, since the geometrical singularities of the structure appear in a visual form and are not confused with a defect. In some embodiments, the reference state is no longer necessary.

Industrial structures are often very complex geometrically (due to stiffeners, rivets, collages, ...) and consequently appear a multitude of ultrasound echoes. An imagery therefore greatly helps in the interpretation of signals.

Moreover, a multiplicity of sensors increases the resolution of the imaging and thus enhances the advantages of the invention.

Advantageously, the size of the apparatus according to the invention remains limited, even with many sensors, which allows a relative portability, compatibility and utility with regard to the constraints of an integrated health control system of structures {Structural Health Monitoring, SHM) The measurements are carried out passively, that is to say without emission of acoustic waves. As a result, the energy consumption is reduced and allows embedded solutions (for example on board an aircraft, a boat or at the bottom of the sea).

The method produces a map of the area to control easily interpretable (which limits the risk of false alarms). The method is all the more effective when the acoustic field is diffuse, that is to say that geometric elements diffract multiple times the acoustic field, which is particularly true in industrial structures that are never simple plates but include for example stiffeners, rivets or local extra thicknesses that diffract the waves and enhance the diffuse nature of the field.

The size is reduced, compatible with an integration of the equipment in the structures to be monitored. For example, for integration into composite materials, whereas the use of piezoelectric transducers generally requires two electrical wires per piezoelectric transducer, a single integrated optical fiber between the composite plies has tens of measuring points. The number of entry points into the structure is therefore very limited, which further limits the potential points of weakness. Thus, the invention will find application for Structural Health Monitoring (SHM) structural health monitoring operations, such as for the detection (and characterization) of corrosion on aircraft fuselage, delamination in composite structures , corrosion or deposit in pipes. Advantages related to the embodiments and use of optical fibers include a small footprint, reduced mass, high bandwidth, large offset, electromagnetic immunity, good resistance to severe radiation or ionizing, among others.

Description of figures

Various aspects and advantages of the invention will appear in support of the description of a preferred embodiment of the invention, but not limiting, with reference to the figures below:

Figure 1 shows the diagram of an exemplary device according to the invention;

FIG. 2 illustrates another example of configuration of the measuring points according to the invention;

FIG. 3 illustrates an example of amplitude measurement as a function of the angle of incidence on a sensor;

FIG. 4 illustrates an exemplary configuration of the FBG sensors in a so-called "rosette" form. FIG. 5 illustrates another example of configuration of the optical fiber according to the invention, arranged "in meanders";

Figure 6 illustrates an example for which acoustic rays are impacted by a corroded area; Figure 7 illustrates the possible acoustic paths for an exemplary configuration with 1 6 sensors arranged in a circular fashion.

Detailed description of the invention

The invention can advantageously use a large number of measurement points to be able to perform guided wave tomography. For this, one or more optical fibers on which Bragg gratings (FBG) are etched are embedded in (or glued on) the structure. A single optical fiber may comprise tens of FBGs, and therefore as many measurement points. The size is reduced.

However, FBGs can only operate as a guided wave receiver but not as a transmitter. Current SHM systems based on FBGs therefore use additional piezoelectric transducers as transmitters. To make the tomography would be a piezoelectric transducer by FBG, so always a large number of piezoelectric transducers. According to one embodiment of the invention, a technique is described which makes it possible to provide images of industrial type structures on localized zones and / or of limited thickness (plate or tube type geometry). The images can in particular indicate the propagation speeds of the guided waves. According to some implementations of the invention, this image supply is passive (i.e. without emitting ultrasonic waves by the onboard system).

The method comprises in particular: an ultrasonic field measurement passively, by a diffuse field correlation technique, technique resulting from the geophysics, and recently studied in the context of integrated health control. This type of measurement has never been performed with optical fibers equipped with Bragg gratings (FBG) as sensors. However, it is found that the Bragg gratings (FBG) advantageously allow to have a large number of measurement points.

- a guided wave tomography structure imaging that exploits the presence of this large number of measurement points. Such an imaging is known from the state of the art but only by means of "active" measurements, that is to say measurements requiring the use of ultrasonic wave emitters (for plates or for pipes),

Some embodiments of the invention provide for the use of FBG sensors instead of the piezoelectric transducers commonly used in SHM systems.

The passage of piezoelectric transducers (for example PZT type) to sensors (or measuring points) FBG is not obvious because they are two very different and non-interchangeable technologies. Piezoelectric transducers can be used both as transmitters and as receivers whereas FBGs can only be used as receivers. Moreover, the piezoelectric transducers are omnidirectional whereas the FBGs are directional. Finally, the montages are specific in both cases (electrical vs. optical). Piezoelectric transducers and FBGs are often presented as complementary (transmit piezoelectric transducers and FBGs in reception) and current SHM systems based on FBGs all use piezoelectric transducers embedded in or placed on the structure with the optical fiber as the transmitter of acoustic waves. FIG. 1 shows a possible diagram according to an exemplary embodiment of the device according to the invention. In the example, the device consists of an optical fiber 100 integrated in a structure to be studied (or bonded to its surface) and which comprises a number of Bragg gratings (Fiber Bragg Grating, FBG), such as the measuring point FBG 101, or FBG 102. The measuring points are represented by small rectangles. Figure 4 details a possible configuration of a measurement point by FBG (so-called "rosette" configuration). A single optical fiber generally comprises a few tens of FBG measurement points per fiber or even a few hundred. The optical fiber is embedded or bonded or laid or attached or associated with the structure

According to another embodiment of the invention, several optical fibers can be used. In this case, these fibers are interrogated separately by means of a multiplexer.

The optical fiber 100 is coupled by a coupler 110 to a light source 120 (laser or broadband), which will emit into the fiber, and to a photodetector or an optical spectrum analyzer 130 which will analyze the reflected light. after traveling through the optical fiber, itself connected to a digital processing unit (140). The different acoustic paths in the zone to be inspected 150 passing through the measurement point FBG 101 and each of the other measurement points is illustrated by the acoustic paths 1 60.

There are at least two possibilities for measuring guided waves using FBG measurement points. A first embodiment uses a laser whose wavelength is varied. A second embodiment uses a broadband optical source whose reflected optical spectrum is determined. The first embodiment has the advantage of improved sensitivity. The second embodiment has the advantage of a cost saving.

According to alternative embodiments, the coupler 110 may be replaced by an optical circulator ("circulator" in English, not shown) and the spectrum analyzer (expensive equipment) or the photodetector 130 by FBGs arranged on multiplexed optical fibers ( configuration sometimes called High-Speed Optical Wavelength Interrogation System). Other systems for implementing multiplexed optical fibers exist.

FIG. 2 illustrates another example of configuration of the measuring points according to the invention. The measurement points FBG (101, 102, ...) can be arranged in different ways around the area to be inspected 150. FIG. 2 illustrates another spatial configuration of the optical fiber 100 carrying the FBG 101, 102, etc. The different configurations in terms of layout and number of measurement points are only limited by those resulting from the performances of subsequent reconstruction, by means of the tomography algorithm chosen for the defect which one wishes to study.

Figure 3 illustrates an example of amplitude measurement as a function of the angle of incidence on a sensor. FBGs in themselves are directional sensors: the measured amplitude depends on the angle of incidence of the wave on the sensor (Figure 3a). The fiber 100 comprises an FBG sensor 310 oriented at an angle alpha 304, exposed to a wave in a direction 301 (perpendicular 302): the amplitude is maximum (305) when the FBG is in the direction of propagation of the wave and zero or minimal (306) when it is orthogonal to it (Figure 3b). Since the FBGs are etched in the axis of the optical fiber, if the arrangement presented in FIGS. 1 and 2 corresponds to the actual orientation of the FBGs, the measured amplitude would be practically zero for all the pairs of FBGs of interest. for those whose acoustic path crosses the heart of the area to be inspected. In a particular embodiment of the invention, omnidirectional type optical fiber sensors (for example of the "FOD" Doppler effect-based fiber optic type) are used (instead of or in addition to FBG sensors).

In another embodiment, a so-called "rosette" configuration is used, illustrated in FIG. 4. The figure shows the detail of the arrangement of each measuring point, for example the measuring point FBG 101, the various points 1 and 2. Each measurement point comprises three FBG arrays arranged at 120 ° to each other (FBG 1 401, FBG 2 402, FBG 3 403). Because of this spatial configuration, for each pair of measurement points the correlation is made between the two FBGs (one for each measurement point) which are the best aligned.

According to a variant illustrated in FIG. 5a, the optical fiber 100 may be arranged according to "meanders". In this configuration, fewer paths are then available for tomography (only those for which the FBGs are relatively well aligned are used, in the figure the marked paths 501).

For each pair (A, B) of measuring points of this network, a correlation of the acoustic field u measured simultaneously over a long period in A and B is performed, for example by applying (there are other possibilities of calculation): Qs (= / u _A (t + τ). Ϊ́ί _Β (τ) άτ.

It is established that the correlation (in all rigor its derivative) converges towards the function of Green between A and B if the various components of the wave field respect the condition of equi-distribution in energy (the distribution in phase and in amplitude waves is random, so-called "diffuse field" hypothesis). The Green function between A and B is the record that would be obtained in A if a source emitted a Dirac in B.

The equi-energy distribution conditions can be obtained when the sources are randomly distributed in the medium or when the number and distribution of sources is limited but the medium is very diffusive. Experimental demonstrations have shown that convergence is achieved in interesting frequency ranges for SHM (from kilohertz to a few megahertz).

For example, natural noise sources in industrial structures may be those associated with the turbulent boundary layer in aeronautics, wave impact, engine-induced vibration on a ship, or turbulent flow in a tube.

According to a variant illustrated in FIG. 5b, the optical fiber can be arranged without particular meanders (which may be easier or feasible in certain situations). One way to overcome this relatively unfavorable geometry is to carry out the correlation coda correlation which amounts to passing, for each pair (A, B) of measurement points, by at least a third measurement point C, and to make the correlations C _A c and C _B c then the correlation of the coda of these two signals to obtain CAB, this step being able to be repeated for all the measurement points C different from A and B then averaged to improve the signal-to-noise ratio. This implementation requires a simplified arrangement of the fiber, which no longer requires fiber meanders to align the FBGs with respect to each other (FIG. 5b). In return, the signal processing time is longer. In practice this is done in the following way: for the pair considered (A, B) is used another measurement point C, among the set of available points. First, the signals measured between A and B on the one hand and C on the other hand are correlated. Once the correlations C, A and C, B are made, we correlate the coda of these signals to obtain the correlation between A and B. This can be repeated on some or all of the measuring points C ,, we can sum the set of correlations obtained to obtain a better estimate of the Green function between A and B. From the Green function obtained by the correlation, the measurement of the flight time between A and B is deduced. Repeated for all pairs of receivers This operation provides a large amount of time-of-flight data that can be used to perform velocity-domain tomography reconstruction.

FIG. 6 illustrates an example for which the acoustic rays are impacted by a corroded zone 610 on a study zone 150. In the example, certain acoustic rays passing through the measurement points FBG 1 101 (generally FBG n) are impacted. . Of all the possible paths, only those passing through the corroded area 610 (or other damage such as delamination) are impacted (or affected, confer the solid lines in the illustration), the other paths are unchanged (dashed lines). tomography method according to the invention reverses the set of measured flight times, in order to reconstruct a speed chart of propagation compatible with all flight times. For guided waves, the propagation velocity depends on the thickness of the structure (by a known relation, ie the dispersion curves), this propagation velocity map can be transposed into a thickness map if one seeks to detect corrosion. This method also works, for example, to detect the delamination of a composite structure (since at the delamination level the speed of the guided waves is also modified). The map that we obtain is an image of the structure. This image is interpretable: the extent of the damaged area is made visible. For corrosion damage, for example, it becomes possible to know the extent and the residual thickness. As a result, the severity of the damage can be assessed, if necessary to take corrective action.

Obtaining an image of the structure thus makes it possible to detect one or more defects, without the need to subtract the measured signal at a given instant from the one measured at a time t ₀ , a reference state for which the structure is considered to be healthy. The prior provision of this reference state involves many constraints (for example, the need to build a database with measurements at all the temperatures that the structure would have to undergo, problems in the event of aging of the sensors causing false alarms, etc.)

Figure 7 illustrates the possible acoustic paths for an example configuration with 1 6 (type 101) sensors arranged in a circular fashion. The robustness of the method presented here comes from the number of measuring points and therefore the number of possible paths. Figure 7 shows the multiplicity of acoustic paths 1 60 in the case of employment of 1 6 sensors or measuring points. It is possible to use hundreds of sensors.

Different embodiments are possible to implement the tomography, in particular as to the calibration of the method. Tomography requires precise knowledge of the position of FBGs.

According to one embodiment, the individual positions of the measurement points FBG are measured.

According to another embodiment, a calibration is performed just after laying the fiber, at a controlled temperature, in order to measure the flight times between each pair of FBG. If the speed is known, which is not always the case, it is possible to deduce the position of the FBG with a very good accuracy. If not, it is possible to measure the flight time for each of the pairs of sensors and to make a mapping of variation of the propagation speed compared to the initial state. Knowing the temperature at the time of calibration, if we know the temperature of the structure using an integrated thermocouple at time i we can, in addition, compensate the variation of flight time induced by the temperature. Failing this, the temperature usually implies a uniform (although potentially anisotropic) effect whereas a defect will generally have a localized effect. The problems mentioned previously on subtracting the reference state are therefore less critical than in current techniques and above all are offset by a large number of measurement points. According to another embodiment, a mapping of the structure to a healthy state is carried out (reference state of the structure). In this case, there is no need to subtract the signals. This mapping in the healthy state provides an image that makes it possible to identify certain geometric features (such as rivets for example) within the area to be controlled so as not to identify them as defects during subsequent mappings.

According to a completely optional variant, attenuation tomography is performed. The correlation makes it possible to reconstruct not only the phase of Green's function but also its amplitude. Attenuation tomography can then be performed. The convergence of the correlation will be different and the directivity of the FBGs can be compensated. This configuration is advantageous in certain situations, in particular when the defect which one seeks to study has little influence on the speed of propagation of the ultrasonic waves.

According to a development of the invention, the correlation between two FBG located on the same fiber can be performed. According to another development, several optical fibers are used, with correlation between two different FBG located on different fibers.

The present invention can be implemented from hardware and / or software elements. It may be available as a computer program product on a computer readable medium. The support can be electronic, magnetic, optical or electromagnetic.

Claims

claims

1. Method for analyzing a diffuse acoustoelastic field correlation structure, an optical fiber comprising a plurality of measurement points, a measurement point comprising Bragg grating network sensors FBG, the optical fiber being deployed in or on the structure to be analyzed, the method comprising:

the emission of light in the optical fiber;

interrogating at least a portion of the FBG sensor pairs measuring, over time, the acousto-elastic waves at the measurement points;

the correlation measurement for at least a portion of the pairs of FBG sensors of a diffuse acoustoelastic field propagating in the structure.

2. Method further comprising a step of reconstructing the propagation speeds by tomography, the imaging being performed by inverting all the flight times between the FBG sensors, each flight time for each pair of FBG sensors being deduced from the correlation measure.

3. Method according to claim 2, wherein the position in the space of each measuring point is previously and individually measured.

4. The method of claim 2, wherein the temperature of the structure is measured and a change in flight time induced by a temperature change is compensated.

The method of claim 2 including a first measurement performed in an initial or reference state of the structure and comprising an imaging of the structure made by tomography from said first measurement to identify certain geometric features of the structure.

The method of claim 5, further comprising a second measurement performed in a subsequent state for the same pairs of measurement points as the first measurement and further comprising a tomography mapping of propagation velocity changes in the structure between the initial state and the subsequent state obtained from differences in flight times measured between the two states.

7. Method according to any one of the preceding claims for which a measurement point comprises an FBG sensor.

8. A method according to any one of the preceding claims for which a measurement point comprises three receivers and directional FBG sensors substantially disposed at 120 ° from each other in a rosette configuration.

9. The method of claim 1, the correlation measurement comprising a coda correlation correlations between FBG sensors.

A method according to any one of the preceding claims comprising a plurality of optical fibers according to claim 1, each FBG sensor being separately interrogable.

11. System for analyzing a structure, comprising:

at least one optical fiber comprising a plurality of measurement points, a measurement point comprising one or more Bragg grating network sensors FBG;

a light source coupled to the optical fiber; a photodetector or an optical spectrum analyzer for analyzing the reflected light after it has traveled through the optical fiber;

signal processing means for performing correlation calculations of the acoustoelastic field and tomography.

12. System according to the preceding claim, wherein the light source is a laser whose wavelength is varied or a broadband optical source whose reflected optical spectrum is determined.

13. System comprising a plurality of optical fibers according to any one of the two preceding claims, the optical fibers being multiplexed by means of at least one optical circulator and / or a spectrum analyzer and / or a multiplexer.

14. System according to any one of claims 11 to 13 for which one or more unidirectional sensors type FBG are complemented by one or more sensors.

15. System according to any one of claims 11 to 14, further comprising one or more active noise sources positioned in or on the structure so as to obtain a diffuse acoustoelastic field.

6. System according to the preceding claim, at least one active noise source being a piezoelectric transducer.